Adsorption of the Ir₄ cluster on single-wall carbon nanotubes: the zigzag types are more suitable

Abstract

Density functional calculations have been performed to investigate the adsorption of the Ir₄ cluster on different SWNTs. We find that the tetrahedral isomer adsorbed on the studied tubes, with a triangular face on the SWNT, is predominantly favored. The metal–SWNT interaction in the zigzag tube-supported Ir₄ systems is significantly stronger than that in the armchair-supported systems. A strong hybridization between the p orbital of the SWNT and the d orbital of the Ir atoms is formed when the Ir₄ cluster is adsorbed on the SWNT surface. The charge density difference also shows that a substantial concentration of electrons accumulates as Ir–C bonds. The CO/NO adsorption on the SWNT-supported Ir₄ cluster is also investigated. All of the zigzag tube-supported Ir₄ clusters possess lower adsorption energies relative to the free- and armchair-supported Ir₄ clusters. Our investigation indicates that the zigzag-type SWNTs not only stabilize the Ir clusters, but also enhance their catalytic performance.

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Introduction

Several 5d transition metal (TM) clusters, such as Pt, Au and Ir, are effective catalysts and present interesting cluster size effects on catalytic reactivity.^1–5 The studies on iridium as a catalyst were limited to little reactivity in experiments,^6–11 due to the high cost of the Ir resource. In experiments, the shape and stability of Ir_n clusters, with n = 18–39 on the close-packed Ir (111) plane, were studied using a low-temperature field ion microscope by Wang et al.¹² to characterize the energetics of surface clusters. They reported that the high-coordination hexagonal clusters, like Ir₁₉ and Ir₃₇, are far more stable than the less compact ones. The extended X-ray absorption fine structure (EXAFS) measurements of fully decarbonylated systems show that the Ir–Ir coordination number is three, suggesting that the tetrahedral Ir₄ frame of the precursor is preserved. The catalytic behaviors of tetrairidium clusters on solid supports (MgO) were investigated by Xiao et al.¹³ with infrared spectroscopy and chemisorption measurements, and they found that more O₂ molecules were chemisorbed on the solid-supported Ir₄ than on metallic iridium. The low uptake of hydrogen and CO on nearly uniform Ir₄ clusters, in comparison with those for adsorption on metallic iridium, show that the chemisorption properties of the extremely small clusters are different from those of the highly dispersed metal particles. The structural and catalytic characterization of oxide-supported (γ-Al₂O₃, MgO and La₂O₃) iridium nanoclusters was studied by extended X-ray absorption fine structure (EXAFS) spectra.¹⁴ The EXAFS data indicate that the metal clusters, while remaining intact and maintaining their bonding to the supports during catalysis, underwent slight rearrangements to accommodate reactive intermediates.

The adsorption and activation properties of small Pt, Au and Ir clusters towards NO molecules were investigated by Endou et al.¹⁵ with a DFT approach. They found that the order of the energetic stability of the adsorption states of NO is Ir > Pt > Au, and is dependent on neither the geometries of the pentamers, nor the cluster sizes considered in the reference. CO, O₂ and NO molecules and the cubooctahedral model of the Ir₁₃ cluster were selected as the adsorbates and model cluster, respectively, to investigate the catalytic reactivity of Ir₁₃ clusters.¹⁶ The charge transfers between the surface atoms of the cubooctahedral Ir₁₃ cluster and the adsorbates indicated the strong interaction between them. Bussai et al.¹⁷ investigated the structures and binding energies of various isomers of the Ir₄ cluster and its complexes with one H atom, based on the relativistic density functional theory. Recently, Stevanović et al.¹⁸ used DFT based on ultrasoft pseudopotentials to investigate the structural properties of the gaseous and MgO (100)-supported Ir₄ clusters. They also investigated the effect of several adsorbates, including H, C, O, CO and OH, on the equilibrium atomic structure of the clusters. The C or CO adsorption significantly influences the relative stability of the Ir₄ isomers. For MgO (100)-supported Ir₄, atomic carbon is able to change the isomer preference from the square to the tetrahedral geometry. As we know, a carbon nanotube (CNT) has a high surface area and high chemical stability. Recently, it has been found to be an ideal support material for metal clusters.¹⁹

In the present paper, we have employed ab initio calculations to investigate the structural and electronic characteristics of the Ir₄ cluster on two types of single-wall carbon nanotube (SWNT) with a chiral angle of either 0° or 30°. The geometric and electronic structures of the Ir₄ cluster on these two types of SWNT were compared and discussed in detail. Also, the CO/NO adsorption on the SWNT-supported Ir₄ cluster is investigated to understand the catalytic properties of supported metal clusters.

Computational details

All spin-unrestricted calculations are carried out utilizing density functional theory (DFT) in the Dmol³ package.²⁰ The exchange correlation energies were evaluated with the generalized gradient approximation Perdew–Burke–Ernzerhof (PBE)²¹ functional. Since the iridium atom has as many as 77 electrons, the all-electron relativistic calculations are time consuming. Therefore, the DFT-based semi-core pseudopotential (DSPP)²² including scalar relativistic effects, which is generated by fitting the all-electron relativistic DFT results, is employed in our calculations. With the double numerical basis set augmented with a polarization p-function (DNP), which has a computational precision comparable with the split-valence basis set 6-31g**,²³ a decent description for the valence electrons of Ir (5s²5p⁶5d⁷6s²) could be obtained. All electron DNP treatments were used for the C, N and O atoms. In the generation of the numerical basis sets, a global orbital real-space cutoff of 4.5 Å is used. The geometry optimizations are performed with the convergence criterion as follows: 10⁻⁵ a.u. for the total energy, 10⁻⁶ for the electron density, and 2 × 10⁻³ and 5 × 10⁻³ a.u. for the gradient of force and the atomic displacement, respectively. We chose the octupole scheme for the multipolar expansion of the charge density. The thermal smearing (0.002 a.u.) was applied to the orbital occupation to speed up the self-consistent field (SCF) convergence. The Brillouin-zone sampling was restricted to the Γ-point in the geometry optimization, and a 1 × 1 × 28 k-point mesh was used in the property calculations.

Six single-wall carbon nanotubes (SWNT), including zigzag tubes, (7, 0), (8, 0), and (10, 0) and armchair types, (7, 7), (8, 8), and (10, 10) are considered in our calculations to discuss the differences in the adsorption of the Ir₄ tetramer. The studied tubes, which were placed in a rhombic supercell with dimensions of a = b = 30 Å, and c = 4c₀ (c₀ is the lattice parameter of the SWNT studied), were large enough to avoid the interaction between the Ir₄ tetramer and its periodic images. In the NO and CO adsorption cases, the supercell dimensions perpendicular to the tube axes are extended to a = b = 35 Å for the (10, 10) tube.

The relaxed isomers of the Ir₄ tetramer adsorbed on the tubes considered have been optimized without any symmetry constraints. Different adsorption sites were considered in the geometry optimization. The initial adsorption sites of the NO and CO molecules on the supported Ir₄ were determined based on the Fukui function, and then the molecularly adsorbed systems are fully relaxed without symmetry constraint. The formation energy of the SWNT-supported Ir₄ system was calculated as the energy difference between the SWNT-supported Ir₄, and the isolated SWNT and Ir₄ cluster. As for the adsorption energy of CO/NO on the SWNT-supported Ir₄ system; it is defined as the energy difference between the molecularly-adsorbed SWNT–Ir₄ system, and the free molecules and SWNT–Ir₄.

Results and discussion

Adsorbed structures and stability

For the free Ir₄ clusters, there are three isomers found in the geometry optimization, and the square (S) structure with an Ir–Ir bond length of 2.40 Å is most favorable in energy. This favorable isomer corresponds to a spin magnetic moment of 2.0μ_b per atom. The nonmagnetic tetrahedron (T) isomer with a bond length of 2.53 Å, corresponding to a relative energy of 0.17 eV, comes next in terms of energy (the relative energy is defined as the energy difference relative to the lowest energy isomer). One butterfly-like structure was found to be less stable than the square and tetrahedron isomers. Our calculated geometry parameters and energy orders for these three isomers are in excellent agreement with previous relativistic DFT calculations.¹⁷ By considering the low stability of the butterfly-like structure, only the square (S) and tetrahedron (T) isomers adsorbed on SWNTs are investigated in the present work. Three different adsorption modes, depicted in Fig. 1, are considered for every type of SWNT in the geometry optimizations of the SWNT-supported Ir₄ system: (i) one Ir atom directly bonding to the tubes, labeled as T-1 for the tetrahedron and S-1 for the square in Fig. 1, (ii) two Ir atoms directly bonding to the tubes, labeled as T-2/S-2, and (iii) three or four Ir atoms directly bonding to the tubes, labeled as T-3 and S-4, respectively. Therefore, there are as many as 36 structures that are relaxed in the optimization. Among these structures, the relaxed structures with the corresponding coordination of the supported tetrahedron isomer as T-3 are given in Fig. S1 of the ESI.† The relative energies calculated for all 36 SWNT-supported Ir₄ systems are depicted in Fig. 2, from which one can clearly see that the tetrahedron isomers adsorbed on the studied tubes in the T-3 mode are predominantly favored in energy. This result is in line with an experimental conclusion in which the tetrahedron shape corresponding to a coordination of 3 was suggested to be exhibited in a supported environment.²⁴ On the other hand, the stability of the square structure, which is most stable in free conditions, is significantly decreased. Except for the T-3 isomer, a great competition exists between the stability of the other isomers of the SWNT-supported Ir₄ system. The existence of many energetically competitive isomers implies that structural fluxionality of the Ir₄ cluster might occur to some degree on the SWNT support. The armchair-supported tetrahedron structure with the T-2 adsorption mode also possesses considerable stability. To further investigate the adsorption dependence of the Ir₄ cluster on the two types of SWNTs, the formation energies (E_f) of the supported SWNT–Ir₄ systems are evaluated using the expression E_f = E_Ir4 + E_SWNT − E_SWNT–Ir4, where E_Ir4 and E_SWNT represent the total energy of the isolated Ir₄ cluster and SWNT, respectively, and E_SWNT–Ir4 represents the total energy of the supported system. The calculated formation energies were depicted in Fig. 3. The detailed data can be found in Table S1 in the ESI.† The formation energies of the square isomer supported on the SWNT are in the range of 2.99–4.82 eV, which are much lower than those of the supported tetrahedron structure ranging from 3.58–5.85 eV, identifying the weaker interaction between the SWNT and the square structure. This result is consistent with the prediction of the relative energy discussed above. As Fig. 3 shows, it is important to note that the formation energies show obvious differences for the two types of SWNTs studied. The zigzag-supported Ir₄ systems (for the two isomers studied) correspond to significantly larger formation energies relative to the armchair-supported systems. Taking the T-3 structures as an example, the formation energy of the zigzag (10, 0)-supported tetrahedron is 5.85 eV, which is about 1.3 eV larger than the largest value for the armchair-supported one (4.57 eV for the (7, 0)-supported tetrahedron). This means that the interaction between Ir₄ and the zigzag SWNT is much stronger than that with the armchair tubes. Moreover, the interaction energies between the tetrahedron isomer and the zigzag tubes in the T-3 mode are larger by 1–2 eV than that of 3.82 eV in the MgO (100)-supported tetrahedron system, studied previously at the DFT level of theory.¹⁸ In this sense, these zigzag SWNTs are more suitable to be used as substrates for the Ir cluster assembly. To quantify the structural deformation of the Ir₄ cluster after being deposited on the SWNT substrates, we depict in Fig. 4 the formation energy versus the average Ir–Ir (left in Fig. 4) and Ir–C (right in Fig. 4) bond lengths. The average Ir–Ir bond lengths in the most stable isomer of the SWNT-supported tetrahedron structure (T-3) are in the range of 2.536 Å (10, 10) to 2.552 Å (8, 8). When compared with the Ir–Ir bond distance of the isolated tetrahedron isomer, also displayed in Fig. 4, the structure is slightly deformed after being deposited on the SWNT. Also, the structural deformation is found for supported square isomers; taking the S-4 isomer as an example, the Ir–Ir bond lengths, ranging from 2.428 Å (10, 10) to 2.449 Å (7, 0), are slightly elongated relative to that of 2.40 Å for the isolated square isomer. We attribute this structural deformation to the formation of Ir–C bonds, which decreases the degree of undercoordination of the three Ir atoms bonded to the neighboring carbon atoms of the SWNT. From Fig. 4, one can also find that the average Ir–C bonds in the zigzag-supported tetrahedron isomers (T-3) are in general slightly shorter than those in the corresponding armchair-supported isomers, reflecting the stronger cluster–SWNT interaction in the zigzag-supported isomer.

Fig. 1 The schematic representation of the different absorption modes for the square (S) and tetrahedron (T) isomers of the Ir₄ cluster on a SWNT.

Fig. 2 The relative energies of the different SWNT-supported square (S) and tetrahedron (T) isomers.

Fig. 3 The formation energies of the studied SWNT-supported square (S) and tetrahedron (T) isomers versus the curvatures of the SWNTs.

Fig. 4 The average Ir–Ir and Ir–C bond lengths in the SWNT-supported tetrahedron structure (T-3) versus formation energy.

The nature of the interaction between the Ir₄ cluster and the SWNT

Electronic structure analyses based on the projected density of states (PDOS) and the charge-density difference were carried out to facilitate a deeper understanding of the interaction character between the SWNT and the Ir₄ cluster. For the reason of comparison, we will take the (8, 0)-supported tetrahedron and square isomers as an example to identify the bonding nature in the SWNT–Ir₄ system. The total density of states (DOS) was projected on to the p orbitals of the C atoms in SWNT and the d orbitals of the Ir atoms. The summed p DOS of the C atoms and d DOS of the Ir atoms are depicted in Fig. 5; the p and d DOS for the same atoms in isolated SWNT substrates and clusters are also shown in this figure to facilitate an understanding of how the p and d states are modified after the cluster is adsorbed. For the isolated Ir₄ cluster, the tetrahedron (T) and square (S) isomers show significantly different band peaks. The DOS peaks are evidently discrete for the tetrahedron structure as compared to those for the square isomer; this difference stems from the higher T_d symmetry of the tetrahedron structure, resulting in the appearance of a degenerate energy level relative to the D_4h symmetry of the square structure. We also note that the Fermi energy level of the isolated square structure is much lower than that of the tetrahedron isomer; this is responsible for the higher stability of the square structure. By comparing the PDOS of the isolated and the supported Ir₄ cluster in Fig. 5, it is immediately apparent that the d states of the Ir atoms undergo significant broadening upon cluster adsorption. The newly hybrid orbitals can be found in a wide range from −7.5 eV to the Fermi level; this indicates that a strong hybridization between the p orbital of the SWNT and the d orbital of the Ir atoms is formed. This hybridization gives rise to the delocalization and broadening of the d states of the Ir₄ cluster. The d states of the Ir atoms are not only involved in Ir–Ir metal bonds, but also in Ir–C interactions. We also note that the p DOS peaks of the SWNT in the supported tetrahedron structure are clearly shifted by 0.27 eV towards a lower energy level, by comparing with the free SWNT. The energy shift is much greater in the supported tetrahedron structure relative to that in the supported square system (0.13 eV), indicating to some degree the higher stability of the supported tetrahedron system. The PDOS of the armchair-supported (8, 8) tetrahedron isomer, shown in Fig. 6, are also analyzed for comparison. As expect, the d states of the Ir atoms are also broadened due to the formation of p(C)–d(Ir) hybrid orbitals. However, the d broadening is less wide than for the zigzag-supported system (8, 0). Moreover, the energy level shift of the p DOS peaks of the (8, 8)-SWNT system towards the low energy region is less significant than that in the (8, 0)-supported system.

Fig. 5 The projected density of states (PDOS) of the isolated (8, 0) SWNT and Ir₄, and the two (8, 0)-supported isomers (tetrahedron and square) of the Ir₄ cluster.

Fig. 6 The projected density of states (PDOS) of the isolated (8, 8) SWNT and Ir₄ tetrahedron isomer, and the (8, 8)-supported tetrahedron isomer.

The electron deformation density (DD), which is calculated as the density difference between the (10, 0)-Ir₄ system and the isolated C and Ir atoms, is used to investigate the bonding changes of the Ir–Ir chemical bonds after being deposited on the SWNT. The charge-density difference (CDD) is also calculated to probe into the interaction nature between the SWNT and the metal cluster. The CDD is defined as the difference between the charge density of the (10, 0)-Ir₄ system and the sum of the charge densities of the isolated Ir₄ isomers (tetrahedron and square) and the (10, 0) SWNT, Δρ = ρ_SWNT–Ir4 − (ρ_SWNT + ρ_Ir4). Two types of density difference are depicted in Fig. 7 for the (10, 0)-supported tetrahedron and square isomers. From the deformation density (right in Fig. 7), mapped on the total density, it appears that electron accumulation is obviously shown between two Ir atoms. Therefore, the covalent nature of the Ir–Ir bonds is still maintained after deposition on the SWNT. We also note that the electron accumulation among the Ir atoms in the square isomer is more significant than that in the tetrahedron structure, indicating the stronger Ir–Ir bond strength. From the charge density difference (left in Fig. 7), one can find that the electron density is significantly modified for both isomers of the Ir₄ cluster when adsorbed on the SWNT. On the other hand, the C atoms which are not bonded with the metal cluster show an unchanged density distribution. Changes in the electron densities occur mainly at the interface region between the Ir₄ clusters and the SWNT, where a substantial concentration of electrons accumulates as Ir–C bonds.

Fig. 7 Charge-density difference in the Ir₄ isomers deposited on (10, 0) SWNT (left) and deformation density mapped on total density on the cutoff plane of Ir–Ir bonds (right).

CO and NO adsorption on the SWNT-supported tetrahedron isomer

CO and NO molecules are important reactive intermediates in many processes; we have investigated the behavior of CO and NO molecules adsorbed on the SWNT-supported Ir₄ cluster. In the present work, only the SWNT-supported tetrahedron isomer in the T-3 model is considered due to its high stability. We first calculated the Fukui index²⁵ to predict the local adsorption sites of the CO and NO molecules on the SWNT–Ir₄ system. Many previous reports have shown that the Fukui index is successful in predicting the local reactive sites of metal clusters. The radial Fukui index mapped on the total electron density is shown in Fig. 8; as expected, the top Ir atom, which is only bonded to metal atoms, corresponds to a large f⁰(r) value of 0.11, which is significantly larger than the other metal atoms (f⁰(r) = 0.05); therefore, the top Ir atom shows a higher chemical reactivity than the other Ir atoms, bonded to the C atoms of the SWNT. The structures of the SWNT supported tetrahedron (T-3) with CO/NO molecules bonded to the top Ir atom are completely relaxed without any constraints. The key structural parameters are listed in Table 1. The relaxed geometrical structures and the corresponding coordinations of the CO/NO molecule-attached systems are depicted in Fig. S2 and S3 of the ESI, and the corresponding coordinations are also listed in the ESI.†

Fig. 8 The radial Fukui function mapped on the total electron density of the (8, 0) SWNT-supported Ir₄ tetrahedron isomer; the condensed radial Fukui index is also shown.

Table 1 Geometrical parameters: the average distances between Ir and the Ir atoms, Ir and the C atoms of the SWNT, Ir and the adsorbed molecules in the different SWNT-supported tetrahedron isomers with and without CO/NO adsorbed

Substrates	Ir–Ir^a	Ir–CG^a	Ir–Ir^b	Ir–CG^b	Ir–CCO	C–O	Ir–Ir^c	Ir–CG^c	Ir–NNO	N–O
a SWNT–Ir4 system.b SWNT–Ir4–CO system.c SWNT–Ir4–NO system.
Free	2.530		2.540		1.866	1.172	2.542		1.785	1.183
(7, 0)	2.541	1.939	2.558	2.013	2.195	1.166	2.571	2.058	2.155	1.179
(8, 0)	2.540	2.066	2.545	2.050	2.179	1.155	2.568	2.125	2.058	1.173
(10, 0)	2.539	1.745	2.552	1.752	1.712	1.168	2.566	1.841	1.737	1.183
(7, 7)	2.543	2.079	2.556	2.113	2.176	1.175	2.564	2.106	2.115	1.175
(8, 8)	2.552	2.214	2.557	2.222	2.095	1.176	2.572	2.151	1.775	1.179
(10, 10)	2.536	1.868	2.603	2.155	1.821	1.179	2.601	2.175	1.810	1.180

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In CO and NO environments, the Ir–Ir bond distances of the supported cluster are more or less elongated as compared to their free supported counterparts. Also, an elongation of the distance between Ir and the neighboring C atoms in the SWNT is found when CO/NO molecules are attached. The C–O bond lengths are significantly stretched by 0.03 Å after being adsorbed on the cluster. There is, however, no obvious bond stretch found for the NO molecule; this may arise from the strong N–O bond, which can not be easily broken. It is worth noting that the Ir–N_NO bond lengths are generally smaller than those of the Ir–C_CO bond lengths in the same substrate systems. This indicates that the interaction between the NO molecule and the supported cluster would be stronger than the Ir–C_CO interaction. The adsorption energies (E_ads), which were evaluated as the difference between the total energy of the SWNT–Ir₄–CO (NO) system and the total energy of the SWNT–Ir₄ system and the CO (NO) molecule, are summarized in Table S2 of the ESI.† We depict the E_ads values versus the curvature of the studied tubes in Fig. 9. By comparing the E_ads of the CO and NO molecules, it is immediately apparent that the adsorption energies of NO in all of the studied tubes, ranging from 2.82–3.47 eV, are much larger than their CO counterparts, which are in the range of 2.31–2.91 eV. The high adsorption energy of NO is in line with the shorter Ir–N_NO bond length. It is very interesting that the adsorption energy for both CO and NO shows clear differences for the two types of SWNTs studied. All of the zigzag tube-supported Ir₄ clusters possess lower adsorption energies relative to the free- and armchair-supported Ir₄ clusters. Specifically, the adsorption energies of CO on the zigzag SWNT-supported Ir₄, ranging from 2.31–2.52 eV, are significantly lower than the value of 2.83 eV in the free Ir₄–CO system. For the NO molecule, the zigzag SWNT-supported Ir₄ systems possess adsorption energies of 2.82–3.11 eV, which are also lower than the value of 3.15 eV in the free Ir₄–NO system. These results indicate that the catalytic performance of the zigzag SWNT-supported Ir cluster can be enhanced, especially for the CO molecule. More importantly, the Ir₄ clusters are more likely to be deposited on the zigzag tubes from the formation energy analyses above. Therefore, we obtain the important information that the zigzag SWNTs will be more excellent substrates in the catalytic application of Ir clusters. We now perform electronic structure analyses, based on the projected density of states (PDOS), to gain insights into the interaction between the supported Ir₄ cluster and the adsorbed CO/NO molecules. The s/p PDOS of the CO/NO molecules and the s/d PDOS of the bonding Ir atom in the (8, 0)-supported Ir₄ cluster (T-3) are shown in Fig. 10. Under the CO/NO environment, the d orbital of the metal atoms undergoes broadening to be involved in meta-CO/NO bonds. The electronic states of the adsorbed molecules are also significantly modified after being bonded to the metal atoms. There are significant overlaps between the d orbitals of the Ir atoms and the p orbitals (mainly the 2π* antibonding state) and the s orbitals (5σ bonding state) of the CO molecule. Similar overlaps are also formed for the p and s orbitals of the NO molecule and the d bands of the Ir atoms. We also note the orbital coupling between the p/s orbitals of the NO molecule and the d states of the Ir atoms is much stronger than that in the CO-adsorbed system. This can be used, to some extent, to explain the higher adsorption energy of NO in the SWNT-supported Ir₄ cluster.

Fig. 9 The CO and NO adsorption energies on SWNT-supported Ir₄ tetrahedron isomers of different curvatures in the T-3 model.

Fig. 10 The projected density of states (PDOS) of the CO/NO molecule and the supported Ir atoms with and without CO/NO adsorption.

Conclusion

We have systemically investigated the adsorption of the Ir₄ cluster on the armchair and zigzag types of SWNT using ab initio calculations. The relative energies show that the tetrahedron isomer adsorbed on the studied tubes with the triangular face on the SWNT (T-3 mode) is predominantly favorable in energy. The zigzag-supported Ir₄ systems (for the two isomers studied) correspond to significantly larger formation energies, relative to the armchair-supported systems. Therefore, the zigzag type SWNTs are more appropriate to be used as substrates for the Ir cluster assembly. The d states of the Ir atoms undergo significant broadening upon cluster adsorption. The newly hybrid orbitals can be found in a wide range from −7.5 eV to the Fermi level; this indicates that a strong hybridization between the p orbital of the SWNT and the d orbital of the Ir atoms is formed. The covalent nature of the Ir–Ir bonds of the Ir₄ cluster are maintained after deposition on the SWNT. The charge-density difference shows that a substantial concentration of electrons accumulates as Ir–C bonds from the mechanism of charge transfer from the metal atom to the bonding C atoms. The CO/NO adsorption on the SWNT-supported Ir₄ cluster is also investigated. All of the zigzag SWNT-supported Ir₄ clusters possess lower adsorption energies relative to the free cluster and the armchair-supported tubes. This suggests that these zigzag-type SWNTs not only stabilize the Ir clusters, but also enhance their catalytic performance to a certain extent. The analyses of PDOS indicate that there are significant overlaps between the d orbitals of the Ir atoms and the p orbitals of the CO or NO molecule.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (no. 11204193). The author Gang Jiang acknowledges the funding support from the National Natural Science Foundation of China (no. 11174213).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09523a

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